Asphaltenes are polar, polyaromatic macro-molecules that are present in crude oils, along with resins, aromatics and alkanes. Asphaltenes are typically present in crude oils in an amount of from 0.1 to 10% by weight of crude oil, though even higher amounts of asphaltenes may be found in heavy crude oils.
At high pressure, under reservoir conditions, asphaltenes are stable and thought to exist in equilibrium, either as individual molecules or as stable nano-aggregates suspended in the crude oil. However, when conditions change and equilibrium is disturbed, asphaltenes can precipitate and form larger aggregates by flocculation. Flocculation is the point at which precipitated asphaltenes start to grow in size and become unstable.
Deposition occurs when the flocculated asphaltenes are no longer carried with the flow of crude oil and form deposits that adhere to well tubing and pipelines downstream of the wellhead. Deposits from a production well may contain about 70% by weight asphaltene, as well as resins, aromatics and alkanes in smaller amounts.
Asphaltene deposition is a major concern in upstream oil production, transportation pipelines and downstream processing because it can lead to e.g. plugging of wellbores and flow-lines, reservoir impairment, emulsion formation, pump failure and heat exchanger fouling. Thus, asphaltene deposition causes significant operational and economic consequences.
In order to avoid costly well workovers, lost production, and high maintenance cycles, field operators generally employ both mechanical means to remove precipitated asphaltene, and chemical means to prevent asphaltene precipitation and/or deposition occurring in the first place.
Mechanical control of asphaltene-deposition is generally carried out using cleaning pigs. Deposits which are removed from a pipeline using a cleaning pig are known as ‘pigged deposits’.
Chemical control of asphaltene deposition is generally carried out by injecting ppm levels of an asphaltene stabilizer. Asphaltene stabilizers include inhibitors, which slow or prevent the initial precipitation of asphaltene molecules from solution, and dispersants, which slow or prevent the flocculation of precipitated asphaltene molecules into larger particles. Asphaltene stabilizers can be identified in a number of different ways. In some methods, asphaltene stabilizers may be identified and tested by separating precipitated asphaltene from a crude oil, and using the precipitated asphaltene to test asphaltene stabilizers.
For example, US 2011/162558 discloses a method in which a sample of precipitated asphaltene is added to hexane and a dispersant formulation and subjected to agitation in an ultrasound bath. After 6 hours, the amount of precipitated asphaltene is measured and compared with a reference sample to determine the efficiency of the dispersant formulation.
In U.S. Pat. No. 8,177,960, asphaltenic components are separated from a hydrocarbon medium by precipitation with n-heptane. These components are then dissolved in toluene. Asphaltene-stabilizing molecules are added to the solution. An asphaltene precipitant, n-hexane, is added to the samples which are observed over 24 hours for signs of precipitation. The experiments are repeated to determine the amount of asphaltene-stabilizing molecules necessary to keep the asphaltenes in solution.
However, by carrying out these experiments in solvents and by using asphaltene which has been precipitated from a hydrocarbon fluid in the laboratory, results may be obtained which are not reflective of the performance of an asphaltene stabilizer in e.g. a crude oil that is present in a production, transportation or processing pipeline.
In other methods, asphaltene stabilizers are often selected based on laboratory screening methods in which a stock tank oil (also known as “dead oil” or “raw crude”) is used. This is because it is very expensive to obtain live oil and to conduct screening methods using live oil.
For example, Oskui et al in “Laboratory Technique for Screening Asphaltene Inhibitors for Kuwaiti Reservoirs”, SPE 106361 (2006) disclose a method in which the effect of different asphaltene inhibitors was analysed by titrating n-heptane against samples of stock tank oil to which the inhibitors had been added.
However, since stock tank oil may not contain the most problematic asphaltenes, e.g. because they were deposited during oil production and/or deposited as sludge in the stock tank, the use of stock tank oils in methods for screening asphaltene stabilizers may lead to erroneous or inconclusive results.
Accordingly, there is a need for a more accurate method for screening candidates for their efficacy as asphaltene stabilizers.